create table t_str as
select round(dbms_random.value(1e10,9e10)) str from dual connect by level<=1e5
/

with
  function ff(s varchar2) return varchar2 
  as
      type avarchars is table of varchar2(100) index by varchar2(1);
      st  avarchars;
      idx varchar2(1);
      res varchar2(10);
      
      function iterate( idx in out nocopy varchar2, arr in out nocopy avarchars) 
         return boolean
      as --pragma inline;
      begin
         if idx is null
            then idx:=arr.first; 
            else idx:=arr.next(idx);
         end if;
         return idx is not null;
      end;  
   begin
     for i in 1..length(s) loop
        st(substr(s,i,1)):=1;
     end loop;
     while iterate(idx,st) loop
        res:=res||idx;
     end loop;
     return res;
   end;

select min(ff(str)) res
from t_str
/

select min(fstr)
from t_str t
     cross apply (
     select listagg(c) within group (order by 1) fstr
     from (select
            distinct substr(t.str, level, 1) c
           from dual
           connect by level <= length(t.str)
          )
     )
/

SQL> create table t_str as
  2  select round(dbms_random.value(1e10,9e10)) str from dual connect by level<=1e5
  3  /

Table created.

Elapsed: 00:00:00.55
SQL> with
  2    function ff(s varchar2) return varchar2
  3    as
  4        type avarchars is table of varchar2(100) index by varchar2(1);
  5        st  avarchars;
  6        idx varchar2(1);
  7        res varchar2(10);
  8
  9        function iterate( idx in out nocopy varchar2, arr in out nocopy avarchars)
 10           return boolean
 11        as --pragma inline;
 12        begin
 13           if idx is null
 14              then idx:=arr.first;
 15              else idx:=arr.next(idx);
 16           end if;
 17           return idx is not null;
 18        end;
 19     begin
 20       for i in 1..length(s) loop
 21          st(substr(s,i,1)):=1;
 22       end loop;
 23       while iterate(idx,st) loop
 24          res:=res||idx;
 25       end loop;
 26       return res;
 27     end;
 28
 29  select min(ff(str)) res
 30  from t_str
 31  /

RES
--------------------------------------------------------------
0123

Elapsed: 00:00:00.48
SQL> select min(fstr) res2
  2  from t_str t
  3       cross apply (
  4       select listagg(c) within group (order by 1) fstr
  5       from (select
  6              distinct substr(t.str, level, 1) c
  7             from dual
  8             connect by level <= length(t.str)
  9            )
 10       )
 11  /

RES2
--------------------------------------------------------------
0123

Elapsed: 00:00:01.01

with
  function ff(s varchar2) return varchar2 
  as
      type avarchars is table of varchar2(1) index by varchar2(1);
      st  avarchars;
      idx varchar2(1);
      res varchar2(10);
      
      function iterate( idx in out nocopy varchar2, arr in out nocopy avarchars) 
         return boolean
      as --pragma inline;
      begin
         if idx is null
            then idx:=arr.first; 
            else idx:=arr.next(idx);
         end if;
         return idx is not null;
      end;  
   begin
     for i in 1..length(s) loop
        st(substr(s,i,1)):=null;
     end loop;
     while iterate(idx,st) loop
        res:=res||idx;
     end loop;
     return res;
   end;

select min(ff(str)) res
from t_str
/

--drop table t_str_1 purge;
--drop table t_str_2 purge;
--drop table t_str_10 purge;
create table t_str_1 as
select to_char(round(dbms_random.value(1e10,9e10)),'fm00000000000') str from dual connect by level<=1
/
create table t_str_2 as
select to_char(round(dbms_random.value(1e10,9e10)),'fm00000000000') str from dual connect by level<=2
/
create table t_str_10 as
select to_char(round(dbms_random.value(1e10,9e10)),'fm00000000000') str from dual connect by level<=10
/
select 
     (select count(*)     from t_str_1) cnt, 
     (select min(ff(str)) from t_str_1) str, 
     res
from xmltable('fn:min(ora:view("XTENDER","T_STR_1")/ROW/STR/fn:codepoints-to-string(distinct-values(string-to-codepoints(xs:string(./text())))))'
     columns res varchar2(10) path '.'
     )
union all
select
     (select count(*)     from t_str_2) cnt, 
     (select min(ff(str)) from t_str_2) str, 
     res
from xmltable('fn:min(ora:view("XTENDER","T_STR_2")/ROW/STR/fn:codepoints-to-string(distinct-values(string-to-codepoints(xs:string(./text())))))'
     columns res varchar2(10) path '.'
     )
union all
select
     (select count(*)     from t_str_10) cnt, 
     (select min(ff(str)) from t_str_10) str, 
     res
from xmltable('fn:min(ora:view("XTENDER","T_STR_10")/ROW/STR/fn:codepoints-to-string(distinct-values(string-to-codepoints(xs:string(./text())))))'
     columns res varchar2(10) path '.'
     )
/

       CNT STR          RES
---------- ------------ ----------
         1 123457       123457
         2 0123689      0123689
        10 012345678    0568

3 rows selected.

SQL> select * from t_str_10;

STR
------------
55550088506
11455429508
71400096720
38283778061
89880445061
36118993026
89292128423
86133782405
45576175643
56556141220

10 rows selected.

SQL> select *
  2  from xmltable('(ora:view("XTENDER","T_STR_10")/ROW[last()]/STR/fn:codepoints-to-string(for $u in distinct-values(string-to-codepoints(xs:string(./text()))) order by $u return $u))'
  3       columns
  4          res varchar2(10) path '.'
  5       )
  6  /

RES
----------
0568
0568
0568
0568
0568
0568
0568
0568
0568
0568

10 rows selected.

SQL> select/*+ NO_XML_QUERY_REWRITE */ *
  2  from xmltable('(ora:view("XTENDER","T_STR_10")/ROW[last()]/STR/fn:codepoints-to-string(for $u in distinct-values(string-to-codepoints(xs:string(./text()))) order by $u return $u))'
  3       columns
  4          res varchar2(10) path '.'
  5       )
  6  /

RES
----------
0568
0124589
0124679
0123678
0145689
0123689
123489
012345678
134567
012456

10 rows selected.